Summary
Sometimes you may need to right you own custom messages to alert log file. For example when you have set up a "procedure" that moves or compresses one table. Oracle has the dbms_system.ksdwrt procedure to write your message to alert log.

Example

dbms_system.ksdwrt(2,'You custom message');

Will write the text to the alert log.
Use 1 instead of 2 to write to the trace file.
Use 3 to write to both.